1. Field of the Invention
The present invention relates to a pressure sensor for measuring blood pressure, and more particularly, to a pressure sensor used in a tonometry method for measuring blood pressure, and a method of fabricating the pressure sensor.
2. Description of the Related Art
Blood pressure is classified into artery blood pressure, capillary blood pressure, and vein blood pressure. Generally, blood pressure indicates the artery blood pressure that varies according to heartbeat.
In a tonometry method, which is one of the methods of consecutively and non-invasively measuring blood pressure, blood pressure is measured by pressing a pressure sensor in a direction toward a radial artery after locating the pressure sensor on the radial artery of the wrist. A sensing unit of the pressure sensor deforms due to blood pressure, and thus, the blood pressure is measured by measuring the deformation of the sensing unit.
The pressure sensor for measuring blood pressure outputs an electrical signal as a pressure value, and the electrical signal is processed by a signal processing system so as to be read as blood pressure. The pressure sensor and the signal processing system constitute a blood pressure measuring system.
A diaphragm pressure sensor has been disclosed in Japanese Patent Publication Nos. 2000-214027 and 2004-279090. The diaphragm pressure sensor has a structure in which a piezometer is formed on an upper part of single crystal silicon having a (100) crystal surface after a lower part of the single crystal silicon is etched. FIG. 1 is a conceptual drawing of the diaphragm pressure sensor.
Referring to FIG. 1, a single crystal silicon substrate 20 is disposed on a glass substrate 10. The surface of the single crystal silicon substrate 20 has a (100) crystal direction, and a sensing unit 30 is formed by anisotropically wet-etching a lower part of the single crystal silicon substrate 20 using KOH or TMAH. The sensing unit 30 includes piezoresistors 40. A diameter of a radial artery is approximately 1.2 mm or higher.
If the single crystal silicon substrate 20 has a thickness of 500 μm, an angle formed by the anisotropical wet-etching of the single crystal silicon substrate 20 is 54.7 degrees. Thus, a width W of an inclined part formed by etching is approximately 350 μm. Accordingly, if a plurality of pressure sensors are arranged in an array, a dead width between adjacent pressure sensors is approximately 700 μm or higher, and thus, it is difficult to use the pressure sensors as an array pressure sensor. Also, a structure for preventing the sensing unit 30 from excessive deformation is not included. Also, in order to form the sensing unit 30 having a uniform thickness, for example, 10 μm, by wet etching for a long time from a lower part of the single crystal silicon substrate 20, a silicon wafer must have a uniform thickness, and a manufacturing process for forming, for example, wires (not shown) at an upper surface of the single crystal silicon substrate 20 as well as the lower part of the silicon substrate 20, is necessary. Also, the lower part of the single crystal silicon substrate 20 and the glass substrate 10 must be bonded.